Update otel to version 0.21 #97

Job	Run time
cargo check	1m 10s
cargo check (feature combinations)	43s
cargo fmt	6s
cargo test (stable on ubuntu-latest)	1m 3s
cargo check (+MSRV -Zminimal-versions) (1.65.0)	6m 23s
cargo test (beta on ubuntu-latest)	1m 10s
build tests (wasm)	1m 40s
cargo check (+MSRV -Zminimal-versions) (stable)	23s
warnings	1m 5s
cargo test (nightly on ubuntu-latest)	1m 6s
cargo test (stable on windows-latest)	2m 23s
cargo test (stable on macos-latest)	2m 23s
all systems go!	1s
	19m 36s

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